cmake: limit pkg-config to UNIX and MSVC+vcpkg by default

[vszakats]

Limit pkg-config to UNIX and MSVC with vcpkg, by default. Compared to
curl 8.9.1, this unlocks pkg-config on MSVC with vcpkg.

This condition might be updated in the future depending on where
pkg-config can be useful without breaking things. (e.g. to non-cross
MINGW, or all MINGW).

In the meantime everyone is free to override the default and test their
build with pkg-config by setting the CURL_USE_PKGCONFIG=ON CMake
option.

---

Peeking into CMake's built-in Find modules: Around 10% support
pkg-config, some limited to UNIX, some guarded with package-specific
flags, some unguarded:

$ ls -1 Find* | wc -l
     179
$ grep -l -R HINTS Find* | wc -l
      68
$ grep -l -R PkgConf Find* | wc -l
      19

Though we tested the waters in 8.9.1 via the libidn2 dependency (ON by default with
pkg-config), I'm still concerned that unlocking pkg-config almost everywhere (for all
dependencies this time) might be overshooting the target, causing fallouts and manual
opt-out in more cases than ideal.

I also reckon that there has been a lot of changes already around CMake in this release cycle,
so unlocking pkg-config now may be too much to handle at once.

Thus, this PR limits pkg-config to UNIX and MSVC with vcpkg, by default. Compared to
8.9.1, this unlocks pkg-config on MSVC with vcpkg.

Nobody has been publicly complaining about missing pkg-config on their platform.
If anyone does, or we have a future release with fewer changes all around, we might
unlock this by default for more targets (e.g. non-cross MINGW, or all MINGW).

In the meantime everyone is free to override the default and test their build with
pkg-config by setting the CURL_USE_PKGCONFIG=ON CMake option.

Let me know what you think!

edit: pkg-config support does not break curl under MSYS/MINGW. Ref: #14575 (comment)

[talregev]

Let me know what you think!

I agree. Let's hear more people opinion.

[vszakats]

/cc @lazka

[lazka]

I can only speak for MSYS2/MINGW, and using pkg-config there shouldn't be a problem. We've only switched away from autotools a few months ago, so we were using pkg-config before that. But cmake is also fine, as long as it builds :)

[vszakats]

I can only speak for MSYS2/MINGW, and using pkg-config there shouldn't be a problem. We've only switched away from autotools a few months ago, so we were using pkg-config before that. But cmake is also fine, as long as it builds :)

Thanks! I've seen you've switched to CMake :) This is still about CMake, and whether or not to allow looking up dependencies via pkg-config (within the CMake build), by default. Or put it differently, could it break something for MSYS/MINGW badly if we enabled it by default? [ You'll be able to test that with -DCURL_USE_PKGCONFIG=ON with 8.10.0; with current master it's still enabled for MINGW, but I plan to disable it. ]

[lazka]

I just did a rebuild of current master with CURL_USE_PKGCONFIG=ON with our configs for openssl/gnutls/schannel, and all looks good.

[vszakats]

Thank you @lazka!